1. Field of the Invention
The invention relates generally to the field of explosive shaped charges. More specifically, the present invention relates to a composition of matter for use as a liner in a shaped charge, particularly a shaped charge used for oil well perforating.
2. Description of Related Art
Shaped charges are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Shaped charges known in the art for perforating wellbores are used in conjunction with a perforation gun and the shaped charges typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing where the high explosive is usually HMX, RDX, PYX, or HNS. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a xe2x80x9cjetxe2x80x9d. The jet penetrates the casing, the cement and a quantity of the formation. The quantity of the formation which may be penetrated by the jet can be estimated for a particular design shaped charge by test detonation of a similar shaped charge under standardized conditions. The test includes using a long cement xe2x80x9ctargetxe2x80x9d through which the jet partially penetrates. The depth of jet penetration through the specification target for any particular type of shaped charge relates to the depth of jet penetration of the particular perforation gun system through an earth formation.
In order to provide perforations which have efficient hydraulic communication with the formation, it is known in the art to design shaped charges in various ways to provide a jet which can penetrate a large quantity of formation, the quantity usually referred to as the xe2x80x9cpenetration depthxe2x80x9d of the perforation. One method known in the art for increasing the penetration depth is to increase the quantity of explosive provided within the housing. A drawback to increasing the quantity of explosive is that some of the energy of the detonation is expended in directions other than the direction in which the jet is expelled from the housing. As the quantity of explosive is increased, therefore, it is possible to increase the amount of detonation-caused damage to the wellbore and to equipment used to transport the shaped charge to the depth within the wellbore at which the perforation is to be made.
The sound speed of a shaped charge liner is the theoretical maximum speed that the liner can travel and still form a coherent xe2x80x9cjetxe2x80x9d. If the liner is collapsed at a speed (collapse speed) that exceeds the sound speed of the liner material the resulting jet will not be coherent. A coherent jet is a jet that consists of a continuous stream of small particles. A non-coherent jet contains large particles or is a jet comprised of multiple streams of particles. The sound speed of a liner material is calculated by the following equation, sound speed=(bulk modulus/density)xc2xd(Equation 1.1). However, an increased collapse speed will yield increased jet tip speeds. Increased jet tip speeds are desired since an increase in jet tip speed increases the kinetic energy of the jet which in turn provides increased well bore penetration. Therefore, liner materials having higher sound speeds are preferred because this provides for increased collapse speeds while maintaining jet coherency.
Accordingly, it is important to supply a detonation charge to the shaped charge liner that does not cause the shaped charge liner to exceed its sound speed. On the other hand, to maximize penetration depth, it is desired to operate shaped charge liners at close to their sound speed and to utilize shaped charge liners having maximum sound speeds. Furthermore, it is important to produce a jet stream that is coherent because the penetration depth of coherent jet streams is greater than the penetration depth of non-coherent jet streams.
As per Equation 1.1 adjusting the physical properties of the material of the shaped charge liner can affect the sound speed of the liner. Furthermore, this adjustment can be made to increase the maximum allowable speed to form a coherent jet. As noted previously, knowing the sound speed of a shaped charge liner is important since a non-coherent jet will be formed if the collapse speed of the liner well exceeds the sound speed.
It is also known in the art to design the shape of the liner in various ways so as to maximize the penetration depth of the shaped charge for any particular quantity of explosive. Even if the liner geometry and sound speed of the shaped charge liner is optimized, the amount of energy which can be transferred to the liner for making the perforation is necessarily limited by the quantity of explosive.
Shaped charge performance is dependent on other properties of the liner material. Density and ductility are properties that affect the shaped charge performance. Optimal performance of a shaped charge liner occurs when the jet formed by the shaped charge liner is long, coherent and highly dense. The density of the jet can be controlled by utilizing a high density liner material. Jet length is determined by jet tip velocity and the jet velocity gradient. The jet velocity gradient is the rate at which the velocity of the jet changes along the length of the jet whereas the jet tip velocity is the velocity of the jet tip. The jet tip velocity and jet velocity gradient are controlled by liner material and geometry. The higher the jet tip velocity and the jet velocity gradient the longer the jet.
In solid liners, a ductile material is desired since the solid liner can stretch into a longer jet before the velocity gradient causes the liner to begin fragmenting. In porous liners, it is desirable to have the liner form a long, dense, continuous stream of small particles. To produce a coherent jet, either from a solid liner or a porous liner; the liner material must be such that the liner does not splinter into large fragments after detonation.
The solid shaped charge liners are formed by cold working a metal into the desired shape, others are formed by adding a coating onto the cold formed liner to produce a composite liner. Information relevant to cold worked liners is addressed in Winter et al., U.S. Pat. No. 4,766,813, Ayer U.S. Pat. No. 5,279,228, and Skolnick et al., U.S. Pat. No. 4,498,367. However, solid liners suffer from the disadvantage of allowing xe2x80x9ccarrotsxe2x80x9d to form and become lodged in the resulting perforationxe2x80x94which reduces the hydrocarbon flow from the producing zone into the wellbore. Carrots are sections of the shaped charge liner that form into solid slugs after the liner has been detonated and do not become part of the shaped charge jet. Instead, the carrots can take on an oval shape, travel at a velocity that is lower than the shaped charge jet velocity and thus trail the shaped charge jet.
Porous liners are formed by compressing powdered metal into a substantially conically shaped rigid body. Typically, the liners that have been formed by compressing powdered metals have utilized a composite of two or more different metals, where at least one of the powdered metals is a heavy or higher density metal, and at least one of the powdered metals acts as a binder or matrix to bind the heavy or higher density metal. Examples of heavy or higher density metals used in the past to form liners for shaped charges have included tungsten, hafnium, copper, or bismuth. Typically the binders or matrix metals used comprise powdered lead, however powdered bismuth has been used as a binder or matrix metal. While lead and bismuth are more typically used as the binder or matrix material for the powdered metal binder, other metals having high ductility and malleability can be used for the binder or matrix metal. Other metals which have high ductility and malleability and are suitable for use as a binder or matrix metal comprise zinc, tin, uranium, silver, gold, antimony, cobalt, copper, zinc alloys, tin alloys, nickel, and palladium. Information relevant to shaped charge liners formed with powdered metals is addressed in Werner et al., U.S. Pat. No. 5,221,808, Werner et al., U.S. Pat. No. 5,413,048, Leidel, U.S. Pat. No. 5,814,758, Held et al. U.S. Pat. No. 4,613,370, Reese et al., U.S. Pat. No. 5,656,791, and Reese et al., U.S. Pat. No. 5,567,906.
However, each one of the aforementioned references related to powdered metal liners suffer from the disadvantages of liner creep, and/or a high percentage of binder material in the material mix. Liner creep involves the shaped charge liner slightly expanding after the shaped charge has been assembled and stored. Slight expansion of the shaped charge liner reduces shaped charge effectiveness and repeatability.
The binder or matrix material typically has a lower density than the heavy metal component. Accordingly the overall density of the shaped charge liner is reduced when the binder or matrix material possesses a lower density. Reducing the overall density of the shaped charge liner reduces the penetration depth produced by the particular shaped charge. However, implementation of a higher density binder or matrix material will increase the overall density of the shaped charge liner thereby increasing the penetration depth produced by the shaped charge.
The sound speed of the shaped charge liner constituents affect the sound speed of the shaped charge liner. Therefore, increasing the sound speed of the binder or matrix material will in turn increase the sound speed of the shaped charge liner. Since shaped charge liners having increased sound speeds also exhibit better performance by the increased penetration depths, advantages can be realized by implementing binder or matrix materials having increased sound speeds.
Therefore, it is desired to produce a shaped charge liner that is not subject to creep, has an improved overall density, and a high sound speed.
The present invention solves a number of the problems inherent in the prior art by providing a liner for a shaped charge comprising a mixture of powdered heavy metal and powdered metal binder wherein the powdered heavy metal comprises from 50 percent by weight of the mixture to 90 percent by weight of the mixture. The powdered metal binder comprises from 50 percent by weight of the mixture to 10 percent by weight of the mixture. The liner for a shaped charge is formed by compressing the mixture into a liner body. The liner for a shaped charge further comprises powdered graphite intermixed with the powdered heavy metal and the powdered metal binder to act as a lubricant. The preferred powdered heavy metal is tungsten, and the preferred powdered metal binder is a combination of a copper-lead-graphite powder, lead, and molybdenum. Other and further features and advantages will be apparent from the following description of presently preferred embodiments of the invention given for the purpose of disclosure.